Behavior of macOS After Toggling Secure Boot on T2 Chip: Does It Remember the ECID?
Yes, the T2 always "remembers" its unique ID (ECID) even if you turn Full Security off and on again. It is never discarded.

However, this is really not the question you would like to be asking:

The ECID is burned into the T2 chip and cannot change. The actual signature file created when the operating system is installed is stored on your disk drive as an im4m file. A valid signature can only be signed by Apple, and it contains the ECID from your T2 chip, limiting its validity to your specific computer.

The real question you want to ask is if those signature files are retained when turning off Full Security - and the answer is yes, they definitely are.

Every time you have a new signature created (for example when installing a new OS), the system creates a new, uniquely named im4m file on your drive. The old ones are not deleted.

When you disable "Full Security" and set it to "No Security", then the T2 chip stops checking the validity of the signatures in those im4m files. It doesn't discard them, delete them or anything of the sort.