Hello, I have a question about :

Older oblexes, called adults and elders, have eaten so many memories that they can form duplicates of the creatures they have devoured from the substance of their bodies, sending them off to lure prey into their clutches, while remaining tethered to the slime by long tendrils of goo. These duplicated creatures are indistinguishable from their victims except for a faint sulfurous smell. Oblexes use these duplicates to lure prey into danger or to infiltrate settlements so they can feed on superior victims.
Sulfurous Impersonation. As a bonus action , the oblex can extrude a piece of itself that assumes the appearance of one Medium or smaller creature whose memories it has stolen. Th is simulacrum appears, feels, and sounds exactly like the creature it impersonates, though it sme ll s faintly of sulfur. The oblex can impersonate 1d4 + 1 different creatures , each one tethered to its body by a strand of slime that can extend up to 120 feet away. For all practical purposes, the simulacrum is the oblex, meaning that the oblex occupies its space and t he simulacrum's space simultaneously. The slimy tether is immune to damage, but it is severed if there is no opening at least 1 inch wide between the oblex's main body and the simulacrum. The simulacrum disappears ifthe tether is severed.

Is it similar to a simulacrum spell ?

Thanks for your future help.

No. This trait is more like 1 creature being in 2 places at once rather than making a copy of itself.

OK.

When an oblex causes a target to become memory drained, the oblex learns all the languages the target knows and gains all its proficiencies, except any saving throw proficiencies.

So, there are some proficiencies from the impersonated creature but not class features or feats, is it right ?

Quote from Waldham >>

OK.

When an oblex causes a target to become memory drained, the oblex learns all the languages the target knows and gains all its proficiencies, except any saving throw proficiencies.

So, there are some proficiencies from the impersonated creature but not class features or feats, is it right ?

 Correct. Proficiencies are skills, tools (including games and instruments), weapons, armor, and vehicles. It will get proficiencies granted by class features and feats, but not the features/feats themselves.

Hehe I used an Oblex that ran a bar.  Most of the staff were sulfurous impersonations and they used an over abundance of incense in the place to cover up the smell. We still have a hoot as the warlock tried to get with one of the spawn barmaids...

Eat Memories. The oblex targets one creature it can see within 5 feet of it. The target must succeed on a DC 18 Wisdom saving throw or take 44 (8d10) psychic damage and become memory drained until it finishes a short or long rest or until it benefits from the greater restoration or heal spell. Constructs, oozes, plants, and undead succeed on the save automatically.

I suppose that a sentient magic item is not a valid target because it's not a creature ou similar to a construct, is it right ? Same question with a simulacrum from a creature.

Quote from Waldham >>

Eat Memories. The oblex targets one creature it can see within 5 feet of it. The target must succeed on a DC 18 Wisdom saving throw or take 44 (8d10) psychic damage and become memory drained until it finishes a short or long rest or until it benefits from the greater restoration or heal spell. Constructs, oozes, plants, and undead succeed on the save automatically.

I suppose that a sentient magic item is not a valid target because it's not a creature ou similar to a construct, is it right ? Same question with a simulacrum from a creature.

 Correct about items. Simulacrums are creatures but they are constructs.