Because their website says that 15.1 is still supported in general, but my device hasn't received any updates in 2 months (and almost daily before that).
Eventually (lineageos web pages are a little scattered) i found this:
https://www.lineageoslog.com/15.1
enable javascript and choose your device from the left sidebar.
for mine, it said: "The selected device is not currently supported in LineageOS 15.1." - it doesn't get clearer than this.
