Is it on MacOS possible to get notified by eMail if the space on a disk gets low.
Get Alert on low disk space
disk-spaceemailhardware
Related Solutions
mdutil is the command line tool that controls Spotlight.
Looking at the mdutil man page, there doesn't appear to be an option to make the index persistent regardless of disk space. There is mention that "indexing may be delayed due to low disk space or other conditions".
One idea that comes to mind is that you can take away write permissions on your index files for all users, this should prevent them from being deleted, but it would also prevent them from being updated, and a static index is likely not desirable, and could cause errors. This might be worthy of testing, but I don't expect it would go very well.
It doesn't appear that Spotlight is designed to work in the way you are looking for. I would suggest submitting a feature request to Apple.
Check that your are not keeping a copy of every email you've received, with their attachments. Under Mail / Preferences / Accounts, by selecting your account and then Advanced, you'll find an option regarding this matter.
There are several options which may help you saving some space.
Best Answer
Here is a script that will do what you need. Let me know if you need a hand configuring it (although the article goes into detail about configuring it to check at intervals etc)
http://www.macmanager.com.au/software/low_space_script.html