It seems that IMDb doesn't know exactly what to do about trans actors.
For cisgender actors who have appeared under different names than the one they're currently known as, they will include that name in their credits. For example, look at Samuel L. Jackson's page and you'll see a few credits in the 80s and early 90s where it says "(as Sam Jackson)" to acknowledge that he was credited under that name.
Understandably, they don't seem to do that for trans actors with their deadname. But what they do do is inconsistent.
For example, if you look at Elliot Page's page, you will not see "(as ...)" anywhere on his credits from before his transition.
However, if you look at Lux Pascal's page, you'll see "(as a different name)" next to all her credits from before her transition.
So, for Lux, they acknowledge that she was credited under a different name without stating her deadname, but for Elliott, they don't even make that acknowledgement.
I don't know if maybe Elliott asked them to make that change, or they just don't have a consistent policy.
I don't know which option is better, but I am glad that at least they aren't including the deadname.