Ed_Flecko
What others have said regarding the security flaws of an EOL device that is no longer receiving updates is 100% correct, but it seems no one has actually answered your question:
Does that mean that my phone IS capable of installing GrapheneOS?
Yes, carrier-variants will have the OEM unlocking
option grayed out, but since your Pixel 4a5g has the option enabled, you will be able to install GrapheneOS on your device.
With that said, I recommend you to follow GOS security best practices and to use a phone that is not EOL.