The tabAlign token has been reserved for many years, and we've seen so much work on the field object that we now have all sorts of more peripheral things like flagged ranges and the textStyles array, yet we still don't have tabAlign. I don't even see it on the Road Map.
Any list that includes numbers needs this.
May I ask what's prevented it from being implemented, and when it might be?
Nascent, yes. The lextable.cpp file contains a list of reserved words that the LC engine knows about, and associates them with internal tokens which can be used by routines in other files. But the fact that a keyword is reserved doesn't necessarily mean that it has been implemented anywhere. The "TT_PROPERTY" tag means that the keyword "tabAlign" has been reserved to be a property of something, but that particular something may or may not have been assigned yet.
But good catch on finding that in the lexical table.