Just sent 61 messages to admins running very outdated copies of either Mastodon or Misskey.
No one should be using, for example, Mastodon 3.x. There is a vulnerability that allows someone to obtain limited access to the owner account.
A few people were using 4.2.x, which is so old and discontinued that the final release at the time suggested upgrading to 4.3.x — which is also now discontinued and has several known security vulnerabilities.
If you're using Mastodon 4.4.0, as legacy, know that 4.4.24 already recommends you upgrade to at minimum 4.6. or newer, The current release is 4.7.1.
No one is saying you need to always run the bleeding-edge release. But if you're going to host other people, you should attempt to keep your site relatively current. The older the site, the harder it can be to upgrade, so not falling too far behind is good planning. If you cannot do that, you really should consider joining someone else's site as opposed to running your own.