Experimental and lowering to pedantic are orthogonal flags. I think both should be done, as removing --as-needed has very marginal relevance so it can't possibly be labeled as "warning". But due to what Christian said (which matches my thoughts) I likewise believe the tag should be hidden for the time being. I
agree that marking the tag as experimental should be enough. I reckon not many display experimental tags, since those are hidden behind a flag that kind of discourage its usage.
Perhaps also consider adding a note in the description that suitability of the change needs to be considerated if the package will be subject of backporting (whilst it's true that there is a risk of overlinking, many of those are not that severe: think of a Qt application that links a a bunch of Qt library just because upstream cargo-culted a list in a qmake file, even when not using all of it. An extra link to a QtSvg that is not used won't bother anybody. But there are more severe cases.). Such note should clearly mention that this is safe only if the package won't target anything lower than bullseye, so that it can stay also during the bookwork cycle for people considering oldstable-bpo uploads.
Lastly, I am not sure if this works for your development workflow, place a code comment todo to drop the experimental flag once bullseye is out.
Thank you for your tireless work on lintian!