[Nekromantik]

Do nightlies stop for Kodi 18 builds once master changes to Kodi 19?

Yes, mine do as I track LibreELEC and Kodi master.

Bigger changes will now be landing in LibreELEC and Kodi masters now that all the branching for stable releases is complete - OMXplayer support is due to be removed from Kodi which will mean dropping RPi0/RPi1 support in LibreELEC (bye by RPi builds - RPi2 builds will continue), and Nvidia VDPAU support is also scheduled for removal from Kodi (but replaced by Nouveau with VAAPI support in LibreELEC, which may only support software decode for more modern Nvidia GPUs thanks to Nvidia now requiring signed firmware to do anything useful on their hardware).

Now that Kodi 18 is released and in theory stable, there's less demand/need for nightlies although "official" nightlies is something that LibreELEC is working on - producing nightlies requires a considerable amount of compute resources which are generally in short supply. 

Ok.
There are nightlies at http://test.libreelec.tv. I assume these are same as yours, based on Kodi 19?