Thanks Günter! Indeed, I use docutils through nbsphinx, and it is now clear that this is not a bug of docutils but the need for nbsphinx to adapt to the line-length-limit API change. I think that this bug report can be closed. Thanks a lot for your help.
Someone just showed me that this was also reported to nbsphinx: https://github.com/spatialaudio/nbsphinx/issues/549
Line length and titles when compiling notebooks with sphinx