Over the last couple of days, I’ve had issues with Spotlight after I moved on to Mojave. The indexing never appeared to go past a certain point.
After a while, I found this in the logs:
So this apparently came with some tags software I tried out in the past, from http://www.caseapps.com/tags/.
After that, I found this page. So I went ahead and removed ~/Library/Spotlight/_TagsMail.mdimporter, and restarted. After that, the Spotlight index seems to progress as it should.