I had noticed that (dewp's) Geohack's preview OSM map has stopped working, and @Strainu reported the failure of the, eh, pop-up-able OSM map at the top of wiki articles as well. This is due to the Toolforge project's tool "wiwosm" referring to a.toolserver.org, b.toolserver.org and c.toolserver.org which have stopped working as well. I'm not sure since when this does no longer work (cf. T85165).
I don't know what the difference between the three hosts was/is and why https://wikitech.wikimedia.org/wiki/Nova_Resource:Maps lists no instances for the Maps project while individual instance pages as for example https://wikitech.wikimedia.org/wiki/Nova_Resource:Maps-tiles2.maps.eqiad.wmflabs seem to exist, but as that OSM plug-in currently just shows a blank canvas, it should be fixed really soon. This could either be done by setting up the redirects (again?) or patching the "wiwosm" tool.