Any .html that we have added in a given version that didn't exist in a previous version will errantly try to link to the nonexistent old page when using the drop-down (it assumes it'll be there). For example on the page:

• https://mne.tools/dev/changes/v0.20.html

Selecting "1.0" with the drop-down tries to go to a page that didn't exist back then (404):

• https://mne.tools/1.0/changes/v0.20.html

@drammock should we escalate to pydata-sphinx-theme?