I'm sure you have heard it before but I think it is worth repeating.
It is risky to continue using old software that is connected to the internet. iOS (like all other software) has regular severe security flaws that get patched. If you are using an old version you will not get these critical security updates and will be vulnerable to various attacks. These can be triggered by things as simple as viewing a malicious image or navigating to a webpage that contains some malicious content.
For example if someone sends you an image over Matrix and it is rendered by your Matrix client they could exploit the application, possible stealing credentials or encryption keys. This would be possible due to the recent WebP vulnerability https://support.apple.com/en-ca/106366