When entering a text snippet for a homebrewed item's attunement description, the snippet as shown in the character sheet reads "requires attunement by a" as a preamble.

I think this is a poor use of language. Consider the following scenarios:
"Requires attunement by an expression of thanks"
"Requires attunement by eating a sandwich"
"Requires attunement by taking a long rest"

I don't think the "a" was always included before, but I disagree with the change. It is not appropriate in many homebrewed scenarios. As a long-time subscriber, I've loved watching how dndbeyond has grown over the years, but the devil is in the details, and I think this detail should be tuned up. Please remove the "a" from the preamble.

 Here's an example from a homebrewed item in my campaign:
https://imgur.com/a/vwq8eNr

Sincerely,
hotpocketlover

I think you're misunderstanding what the purpose of that field is. It's not a field for the action required to complete attunement (requires attunement by doing X), but for a criteria to be met to able to attune (requires attunement by something that is X).

For example, wand of the war mage says "requires attunement by a Spellcaster"

Hey Davyd, thanks for your reply, in the attunement rules, however, I see the following passage in Chapter 14 of the Player's Handbook Basic Rules:

Attuning to an item requires a creature to spend a Short Rest focused on only that item while being in physical contact with it (this can’t be the same short rest used to learn the item’s properties). This focus can take the form of weapon practice (for a weapon), meditation (for a wondrous item), or some other appropriate activity. 

Does this language not imply that attunement not only potentially has a class/spellcaster requirement, but also a specific action that must be engaged in during the short rest? When homebrewing an item, I assume the attunement field is the only appropriate place to put this "other appropriate activity". Thanks again for your response.

The specific activity to attune to an item is rarely described (in fact, I can't think of a single example), but if it was, it'd be described in the description of the item. The attunement field is exclusively for the requirements you must meet, not the actions you must take. 

If you want to include specific attunement steps, I'd suggest putting them in the description of the item

I found a contradiction in first page of the items that are found to be recreated on the web by Dndbeyond developers, the Arcane Propulsion Arm: https://www.dndbeyond.com/magic-items/arcane-propulsion-arm

It states:

Wondrous Item, very rare (requires attunement by a creature missing a hand or an arm)

Going further, couldn't there be a situation where the "a" should be "an", as in, "requires attunement by an Aasimar missing a hand or an arm". Note also this attunement description is in the attunement field rather than the item's description and has nothing to do with a class or spellcaster feature.

I think this is an extremely simple fix. If the item database needed to be updated, a for-loop could go through each JSON item description and prepend "a " for those with existing language dependent on the "a", while the preamble could be shortened to allow for more generality.

Badge of the Watch is another example that exactly pertains to the situation I'm speaking on: https://www.dndbeyond.com/magic-items/badge-of-the-watch

The item's attunement description reads: 

Wondrous Item, rare (requires attunement by a someone designated by the Open Lord of Waterdeep)

"attunement by a someone" doesn't sound like good English to me, but I speak English as a second language so I am no expert.

I'm also going to agree the "a" should be removed. For example, if an item required attunement by an Artificer, it would use "an" not "a". The system using "a" actually means some items, like the badge Halfwing mentions, are not shown here as they are in the sourcebook.

It actually wouldn't be too difficult to change. The "by a" bit is part of a stationary bit and you can replace this by putting just "by" and you can then run an SQL script for all official items to add "a " to the start of the entries. For the exceptions that should use "an" or omit entirely, you can do those manually as they're very few. Put an update out for everyone to know about this change so they can choose to update their homebrew if they need to (even published ones can be easily updated using the 'create a new version' link). 

Sorted. It should take less than hour for a single person to do this. 

The 'a' does indeed cause some issues. Back when D&D Beyond first launched, every existing magical item with an attunement pre-requisite began with "requires attunement by a...". However since then we have got some items that break that grammatical structure, such as not requiring the 'a', or instead requiring an 'an'.

For the most part however it works as intended, but is one of the quality of life improvements the team is no doubt aware of.