The deadline for most applicants was June 30th. If you missed the deadline, you may still be able to apply. See: late applications.

There are special provisions for people who have a residence card or another document issued under EEA Regulations.

Page 43 of the EUSS Guidance says:

Document or status under the EEA Regulations

Where a person subject to the 30 June 2021 application deadline for EEA citizens and their family members resident in the UK by the end of the transition period has a biometric residence card or other residence document issued under the EEA Regulations which remains valid at that date, they may not realise that, with the end of the grace period, they can no longer rely on an EU law right of residence in the UK and need to obtain UK immigration status under the EU Settlement Scheme.

They can make a late application to the scheme where you are satisfied, in line with this guidance, that there are reasonable grounds for them missing the 30 June 2021 deadline, e.g. because they were not aware that they needed to apply to the EU Settlement Scheme.

PR card holders

If you hold a valid Permanent Residence (PR) document issued on the basis of your family relationship to the EU citizen, you don't need to provide any evidence of family relationship.

Holders of valid biometric residence cards

If you hold a valid Biometric Residence card issued on the basis of your family relationship to the EU citizen, you can use it as evidence of family relationship if it was issued to you on the basis of that relationship and that relationship continues to exist (or did so for the period of residence relied upon).

The right type of card will:
say 'EU Right to Reside’ on the back; and
say ‘Residence card of a Family member of a union citizen’ or ‘Residence card issued under the EEA regulations’ at the bottom.

You’ll need to provide evidence of family relationship if:

your card was not issued on the basis of your family relationship; or
your card was issued on the basis of your family relationship with an EU citizen but you’re now relying on a family relationship with a different EU citizen.

In addition to the card, you also need to provide evidence of:

your family member's identity and nationality (or national ID card); and
continuous residence in the UK of your family member.

If you apply with a valid biometric card, you will not be issued with another card under the EUSS.

If your card expires or you wish to be issued with a BRC under the EUSS, you can apply for one.

See: Getting a biometric card under the EU settlement scheme

See also: Evidence for family members
Source: Evidence of relationship