As noted in the comments, "Compat" means compatible, not "Compact", which I and probably most people will read it as. Even after knowing what it means, it still takes me a mental process to "reverse" the meaning of the 2 versions.
So I strongly suggest changing the name. I don't like "legacy" as suggested in comments, maybe just reversing the names will be better -- "3.0" (fat version) and "3.0 Compact".
For what it's worth, and not to take away from your interpretation of "Compat", that made sense to me at first glance. I'm not sure most people will misunderstand the meaning.
The thing that really suggests renaming jQuery Compat to be just jQuery and jQuery to something is (jQuery+?) for me is this in the blog post:
> use the jQuery-Compat 3.0.0 package. We recommend this version for most web sites, since it provides the best compatibility for all website visitors.
So the plan is to call the version that the majority of people will want something other than plain jQuery?...
TLDR: in order to align with semver in the future, the 1.x and 2.x "versions" are being turned into two packages respectively named "jquery" and "jquery-compat", both at version 3.0.0.
So I strongly suggest changing the name. I don't like "legacy" as suggested in comments, maybe just reversing the names will be better -- "3.0" (fat version) and "3.0 Compact".