Right now, the intelligent-magic-items tag exists, with the tag info:
For questions about intelligent or sentient magic items.
(In D&D 5e, at least, these are referred to in the DMG (p. 214) as sentient magic items.)
However, though the two terms are very similar and refer to the same type of magic item, typing in "sentient" when attempting to add a tag to a question yields no results. Even typing "magic" doesn't show this tag in the list (it's not one of the top 6 tags with "magic" in the name). Only once you start typing the second word in "magic-items" does intelligent-magic-items show up.
It's a relatively uncommon tag, so it's hard to have enough reputation in the tag to suggest synonyms - but I suspect at least some of the reason it's not used even on questions where it's appropriate is because it's hard to discover. I just searched RPG.SE for the term "sentient" and added the tag to several questions that didn't have it; I suspect there may be more but stopped after the first page or two. (I added it to a few more thanks to Hey I Can Chan's link in a comment.)