T366314#9869650 and some following discussion has comments that suggest `floatleft` and `floatright` classes are supported for general use of marking up floating content. I find the documentation around these classes a bit rough, with some actionables in various files and onwiki. (I think there's probably merit to using floatleft/right as a general class per previous discussion in T78176. Non-general use can use TemplateStyles where necessary.)
# https://www.mediawiki.org/wiki/Manual:$wgUseLegacyMediaStyles currently says they are part of the legacy media display (which was started as part of T354740). The relevant classes were originally used at some point for thumb markup, and then Parsoid and/or figure migration for Parser.php changed how relevant elements were styled, so that makes sense.
# Is their use shifting? This documentation should be updated if so.
# If in fact they are still connected to this parameter, it would be wise to disconnect them on the software side if indeed the use is shifting (see also #3/#4).
# This general use should be documented on wiki somewhere, and given the context in which it's used, that should probably be at [[https://www.mediawiki.org/wiki/Recommendations_for_mobile_friendly_articles_on_Wikimedia_wikis#Inline_styles_should_not_use_properties_that_impact_sizing_and_positioning | the mobile recs page # Inline styles should not use properties that impact sizing and positioning]].
# [[https://phabricator.wikimedia.org/source/mediawiki/browse/master/resources/src/mediawiki.skinning/content.thumbnails-print.less | content.thumbnails-print.less]] currently removes their borders. This looks like a now-unnecessary holdover from their use in the thumbnail context and can be removed (after checking that the figure structure is printing whatever is desired here without them)?
# [[https://phabricator.wikimedia.org/source/mediawiki/browse/master/resources/src/mediawiki.skinning/content.thumbnails-common.less | content.thumbnails-common.less]] contains the actual CSS, but if they're now being suggested for general use, they probably shouldn't be living in a file pertaining to thumbnails (see also #1B). (The new CSS looks like it's in [[https://phabricator.wikimedia.org/source/mediawiki/browse/master/resources/src/mediawiki.skinning/content.media-common.less | content.media-common.less ]].)
# [[https://phabricator.wikimedia.org/source/mediawiki/browse/master/resources/src/mediawiki.skinning/content.thumbnails-screen.less$3 | content.thumbnails-screen.less]] still contains a reference to these classes despite them being now-absent.
# Lastly, a minor suggestion to improve their definitions: the margins are currently living separate to the clear/float definitions for some reason (don't want the margins to apply to `tleft/right`? resolved by moving floatleft/right out of the same place), but these objects probably shouldn't have their own margins (or at least left/right margins) when they're below the mobile breakpoint in responsive skins.
I realize this is kind of a grabbag but it all also kind of goes together.