It's from legacy to basic, and it was mostly because of technical debt... we made BLS sigs before the basic/regular one came about. I even think (I'm not 100% sure on this though) Platform relies on basic rather than legacy, if you think about it you can pull an off the shelf module for basic or you can try to build one for the legacy format only Dash uses.... there's tradeoffs on all sides. There's also several backported Bitcoin PRs etcI just used Dash Core v21 as example in case the Github pull request was indeed about BLS serialisation and BLS serialisation only (which i was not sure about). If you say that the Github pull request was indeed only about BLS serialisation, then i believe that to be the case.
What i personally would have preferred to see is a focus only on supporting Evo nodes in Dash Core v19 and a focus only on supporting Dash Platform launch in Dash Core v20. Instead features have been added to both Dash Core v19 and Dash Core v20 that are not strictly necessary to support Evo nodes and the launch of Dash Platform. Having BLS upgraded from basic to legacy does little for the Dash network right now and will only be fully utilized down the road, when Dash Core Group focus on blockchain inter-exchangeability. If the BLS upgrade was pushed back to a later update, i wonder if Dash Core v19 would have been in development as long as it did. In my view (assumption) having added the BLS upgrade caused delay of Dash Core v19 and ultimately a severe bug in BLS serialisation also caused the Dash blockchain to halt.
I do think postponing the activation of Dash Core v19 and taking the time to fully fix the issue was the right action of Dash Core Group. I just think Dash Core Group is putting too many features into large Dash Core updates, often causing a Dash Platform delay and in this case even a Dash blockchain halt.
We do have a lot of developers in the core team, and supporting 4k nodes was actually an easy thing to accomplish compared to other tasks... but those other tasks are important and if we leave them for v21 it will just add more and more technical debt to Dash and have most devs idling for 6-12 months.
I'd personally like to see more & better testing