The only thing it does is dissolve adhesives, it's consumable, only dissolves up 1 square foot (a single flask of sovereign glue can hold enough to coat up to 7 square feet), and an item of lower rarity can do the same while also being MUCH more useful.

Clearly this is an item of LEGEND.

Adhesive is defined at any medium that binds two objects together.  By this definition, you can use this to remove the mortar that holds bricks in a wall together.  

I hate to be that guy, but;
Ackchyually: to quote the sovereign glue: "One ounce of the glue can cover a 1-foot square surface" and that distinction of "surface" means a lot when the solvent says "The liquid instantly dissolves up to 1 square foot of adhesive it touches"
That's not just the 6 surfaces that make up the square, but the entire volume of space inside of it.

Now, that might not seem like a lot, but combine it with two specifically mentioned qualities: Liquid, and Adhesive.
1. Adhesive layers tend to be kinda thin, meaning a lot of area can be taken up with 1 square footage of adhesive material, and adhesives are a large category of materials as Ballantyne pointed out "By this definition, you can use this to remove the mortar that holds bricks in a wall together."
2. Liquids tend to be runny.

This means that pouring some on say; a castle wall (have a wizard's flying familiar smash the vial against the wall?) could potentially destabilize a significant portion of that wall, maybe even all the way down to the ground level, maybe even enough that you no longer need to use complicated siege engines to tear down the wall for an assault. A sufficiently angered goliath barbarian, or a handfull of cheap blasting powder, could now do the trick instead.

A lot of items here seem to have absolutly wonky rarities (like the Create Homunculus spell being 6th level) and that's typically because someone in R&D thought of really creative ways to break the game with it.

Quote from RevokFarthis >>

I hate to be that guy, but;
Ackchyually: to quote the sovereign glue: "One ounce of the glue can cover a 1-foot square surface" and that distinction of "surface" means a lot when the solvent says "The liquid instantly dissolves up to 1 square foot of adhesive it touches"
That's not just the 6 surfaces that make up the square, but the entire volume of space inside of it.

Now, that might not seem like a lot, but combine it with two specifically mentioned qualities: Liquid, and Adhesive.
1. Adhesive layers tend to be kinda thin, meaning a lot of area can be taken up with 1 square footage of adhesive material, and adhesives are a large category of materials as Ballantyne pointed out "By this definition, you can use this to remove the mortar that holds bricks in a wall together."
2. Liquids tend to be runny.

This means that pouring some on say; a castle wall (have a wizard's flying familiar smash the vial against the wall?) could potentially destabilize a significant portion of that wall, maybe even all the way down to the ground level, maybe even enough that you no longer need to use complicated siege engines to tear down the wall for an assault. A sufficiently angered goliath barbarian, or a handfull of cheap blasting powder, could now do the trick instead.

A lot of items here seem to have absolutly wonky rarities (like the Create Homunculus spell being 6th level) and that's typically because someone in R&D thought of really creative ways to break the game with it.

 
You seem to be confusing "square" with "cube."

A 1 foot cube has 6 surfaces. 1 x 1 x 1
A 1 foot square has 1 surface. 1 x 1

A square foot is a unit of area not a unit of volume.

The amount of area 1 square foot of adhesive can cover is exactly 1 square foot. That's 144 square inches.

Admittedly dissolving the mortar in a brick wall is kinda nice.

I ran a quick google search and found this site. Which estimates about 20 square inches of mortar per square foot of brick wall. Taking that into account gives you about 7.2 square feet of wall thus liberated of it's bricks. That'll open up a 2.5 foot x 2.88 foot hole in a wall. Why a medium sized creature could crawl through that. That's pretty snazzy. Though gods help you if that wall is more than a single layer thick.

Still underwhelming for a consumable legendary item. I mean oil of etherealness could also move a creature through a wall by means of granting them the effect of the etherealness spell. You can also fly sorta. And you can't be touched or affected by things on the material plane. For an entire hour. This can get you through many, many brick walls.

Sure, making a hole in a wall is handy because other creatures can go through with you. But then again there are spells specifically for that purpose.
Like Passwall or Stone Shape or even Fabricate if your DM isn't extremely picky about what constitutes "raw materials."

I don't know about you but to me it seems like a spell scroll of Stone Shape would be easier to find than a bottle of universal solvent. Worst comes to worst you could pay a spellcaster to scribe one for you.

I mean in theory you can break through a brick wall down with enough brute force let alone use up a consumable legendary magic item to make a single roughly person sized hole.

"A lot of items here seem to have absolutly wonky rarities (like the Create Homunculus spell being 6th level) and that's typically because someone in R&D thought of really creative ways to break the game with it."

What's R&D?

Research and development

Oh. I thought it was a typo. R is very close to D on the keyboard.

I sort of wish another application of this item was making any items attached or fastened together separate. Like you could apply this to the hinges of a door to magically make the door fall down off its hinges. 

Ngl, im not putting uch effort into writing this but you dont need to remove all the mortar, just get rid of half, hell even a quarter and you significantly lower the structural integrity

Quote from CrimsonHex >>

Ngl, im not putting uch effort into writing this but you dont need to remove all the mortar, just get rid of half, hell even a quarter and you significantly lower the structural integrity

 
That's great. Go ahead and multiply my estimate of hole size by 4. So instead of 7.2 square feet it's 28.8. Hell, let's be generous and round that up to 30 square feet. So we got a 5' x 6' hole or multiple smaller holes.

The rest of my point still stands.

feed this to a mimic

What if I drink this after drinking glue

Quote from Jetballoon23707 >>

What if I drink this after drinking glue

You suffer permanent penalties to INT and WIS for thinking that drinking glue and solvent is a good idea.