Reliable like a page won't be lost. Can mediaWiki be more reliable without extensions?
Topic on Project:Support desk
Once you create a page in MediaWiki, the contents are stored in the database.
The only way a page could be "lost" by MediaWiki is if there is a bug that causes the deletion. If you run MediaWiki without any extensions, then a bug like that would be very, very, very surprising since people have used MediaWiki to store millions if not billions of pages.
A page could also be lost because of database corruption, but this, too, is extremely unlikely unless you have other problems with your physical computer.
Please provide clear information how you measure how "reliable" some software is.
In general, you can make MediaWiki (or your computer, or your car, or anything else) more reliable by finding wrong behavior and repairing that wrong behavior by making it right behavior.
Are there any bugs that caused the page to be lost in the history of MediaWIki?
Maybe. I'm not familiar with any that have, but if you'd like to know you can search phabricator which contains all the reported bugs. (Here is one that was first reported in 2004 on SourceForge and then moved to Bugzilla and, finally, to Phabricator, for example.)