This is relevant to those British citizens who, because of Covid restrictions, didn't/couldn't travel here to renew their residence permits, or apply for one having previously travelled here to buy (or perhaps look at) a property.
I was reading the EU guidance on the Brexit Withdrawal Agreement.
https://ec.europa.eu/info/strategy/rela … country_en
The key issue is that the EU differentiates between a British person acquiring residence rights under the agreement, and then formalizing those rights by obtaining the appropriate Article 50 residence permit.
And there are two types of implementations: "constitutive" and "declarative". While there are several constitutive countries, most are declarative.
Constitutive countries (e.g. Germany) had a hard deadline (now passed) for formalizing your rights. Declarative countries (e.g. Bulgaria) accept that you may have acquired rights... but haven't yet formalized those rights. What differs between the declarative countries is what documents you need to prove that you acquired some residence rights there (e.g. you bought a house).
Bulgaria does have deadlines, which are mentioned in their online guidance, for applying for your first permit (end of 2020) or renewing your permit (end of 2021). But it does not appear to be a hard/enforced deadline where you're abolutely too late in January 2022. Rather, you can apply late, but they can subject you to a fine (most administrative fines in Bulgaria are quite small).
My lawyer already told me that he'd got a couple of Residence Permits for Brits who applied late. I'd suspected that he was getting special treatment (or giving someone a "present"), but it seems there is a formal justification in some Covid leniency because of this declarative implementation of the withdrawal agreement.
This is just my reading, so I might well be wrong. But I think, at the very least, if you missed your chance because of Covid, it's worth giving it a shot, even if it's the end of 2021 or early 2022.
I have some confidence in my interpretation, as I've checked on Spain and Cyprus which are also declarative. I had my little blue Registration Card and a property in Spain, but not an official Article 50 Residence Permit (as I now have in Bugaria). The Spanish guidance explicitly states that there's no hard deadline for getting my Registration Card exchanged. Similarly, Cyprus also has leeway, so it seems they should recognize my registration certificate and property purchase from 2005. (My flights in 2020 to apply got cancelled because of the Covid, and until now I thought I'd missed the boat.)