has microsoft made it impossible to use libre office in windows 10?

I have used libre office for many years (version 5) and now suddenly it became impossible to save as or just save a file.
I then installed a new version of the program but the problem did not disappear. I am using an insider version of windows and I have installed every new version of windows since october 2014.

Check if you need to tweak Windows Defender settings.

No.
There are issues surely, and MS will not treat the DocumentFoundation as their best friend from now on, but I used all the versions of LibO (partly only for testing) since 4.something on Win10 and did not experience problems of the kind you describe.
You may backup your user profile by renaming its base folder, and try LibO again. Corrupted user profiles can cause very strange errors in rare cases.
I do not know, however, anything about “insider versions” of Win10.

We always use this excellent tool to clear an enormous amount of unwanted cruft and bloatware from windoze 10.

It helps make win 10 usable in an office environment, with latest LO 5.x