When VE toolbars were upstreamed to OOUI it was with the Apex theme which doesn't show any borders by default (only on hover):
At some point this was replaced with the edge to edge square tools of MediaWikiUI (now WMUI), which added vertical borders between all tool groups, however in VE-MW, almost every tool is in its own group (the exception being undo/redo):
Various teams have independently come to the conclusion that these borders just add clutter. First in Flow:
And recently the editing team on mobile: T231815 - the comparative analysis on that ticket shows it is more common to have fewer borders in editing toolbars.
Removing the borders results in a cleaner look that is still mostly visually parseable:
The one case that might be questionable is the "Paragraph" dropdown, where the label becomes slightly disconnected from the dropdown indicator.