Hello.
How can I disable updating JOSM to a new version? I temporarily cannot install new Java, but it refuses to work on my Java 8
It sounds like you are using the JNLP (this is the only thing I can think of that could be perceived as updating in the background).
If so, you want to download and install OpenWebStart. You may have to right-click on the jnlp (for the Open with option) or rename it to have the file extension .jnlpx if the program associated with the file extension is still Oracle Java 8.
If you are not using the JNLP, we have installers for Mac and Windows that bundle Java.
If you just use a .jar file version of JOSM, it won’t automatically update. However - you’ll soon need a version of JOSM that supports Oauth2 properly, and that will need to be a recent one; so I’d fix the reason why you can’t update Java as quickly as you can.