The changes, as I understand them, were for the validation connections from LE to the DNS/web server, so I wouldn't think they'd affect anything related to the API endpoint. But here's the announcements for the change, if it's helpful:
I think test-ipv6.com does an MTU/MSS test as part of its checks, Cloudflare has a v4 test and a v6 test, and I'm guessing there are more MTU test sites out there too that might help.