Hello there,
Helix Ultimate 2.x.x is compatible with Joomla 3.10 and Joomla 4, however the templates based on the framework are not compatible with Joomla 4 and completely compatible with Joomla 3.10 yet.
If you want to build a fresh website only with the framework, you can use it with Joomla 4. However, do not upgrade to the latest versions on any existing site till the compatibility of the templates. Those will be compatible soon as our dev-team is working on it.
Before doing anything, always take a full site backup for your own safety.
Thanks for being with us.