The package aur/nautilus-megasync 3.6.6-2 is pulling software from March 2020 and it puts the extension in folder
/usr/lib/nautilus/extensions-3
which is obsolete. The new nautilus is looking for extensions in
/usr/lib/nautilus/extensions-4
I have compiled the most recent megasync code and the nautilus extension is working with it. The developers will probably release a new AUR package sooner or later. If you can not wait, you can use my tar file from my previous post.
The package aur/megasync 4.6.8.0-2 works well with the new openssl. It just needs a new compilation