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 ILR to make a late application if they wish to do so.

Page 43 of the EUSS Guidance says:

Existing indefinite leave to enter or remain

Where an EEA citizen or their family member resident in the UK by the end of the transition period has indefinite leave to enter or remain granted another part of the Immigration Rules (or outside the Rules), there is no requirement for them to apply to the EU Settlement Scheme. They may do so if they wish, because for example indefinite leave to enter or remain granted under the scheme (settled status) to an EEA citizen or their family member does not lapse if the person is absent from the UK and Islands for up to 5 years (rather than 2 years as for other forms of such leave). It also provides the person with secure, digital evidence of their immigration status.

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 were eligible for the EU Settlement Scheme.
If you have ILR (Indefinite Leave to Remain), you do not need to apply for settled status, but may do so if you wish and can make a late application as noted above.

You may want to apply for settled status even if you have ILR for the following reasons:

To benefit from enhanced family reunion provisions;
To have a digital status so you can prove your status if you lose your ILR document;
To make it easier to apply for citizenship if you so wish, without having to provide evidence of residence since you were granted ILR decades ago; and
To be able to stay abroad for up to 5 years without losing status.

EU nationals who have lived in the UK since before 2006 may have obtained ILR.

If you are an EEA national and came here on or after 2006, you are unlikely to have ILR.

ILR can be shown as:

A stamp or endorsement on a passport or other residence document such as a residence permit;
A sticker or vignette;
A letter from the Home Office stating you have Indefinite Leave to Enter (ILE) or ILR;
A Biometric document (BRP).

BRPs are most commonly issued to non-EEA nationals.
EEA nationals who applied for ILR under the Windrush Scheme would also have a BRP.

ILR can also be referred to as:

Indefinite Leave to Enter (ILE), if it was granted when you were outside the UK; or
No Time Limit.

See: Sample ILR documents.

If you came to live in the UK before it joined the EU on 1 January 1973, you are very likely to have been given ILR.

If you haven't got the relevant document, you can have it confirmed under the Windrush Scheme.

Applying for settled status with ILR

The application form has the following question:

Click image for larger version  Name:	ILRQuestion.PNG Views:	1 Size:	152.8 KB ID:	1045

If you respond Yes to the above, you will get the following screen:

Click image for larger version  Name:	ILRYear.PNG Views:	1 Size:	83.2 KB ID:	1046
You don't need to provide evidence of residence if you apply with ILR.
If you are unable to provide evidence of ILR, the Home Office will check its own records to see if you were granted ILR in the past.

If the Home Office cannot find any records and you are unable to provide evidence, you will not be able to apply on the basis of having ILR.

If the Home Office finds that your ILR has lapsed due to an absence from the UK of two years or longer, you cannot obtain settled status on the basis of your old ILR.

You may still be able to obtain settled status based on your UK residence.