Is it safe to use AirPort Extreme in 2020 without updates from Apple?

Yes.. with following provisos.

You keep the firmware updated IF apple does release a new firmware. They will release a patch if anything significant is found in the next few years at least.

And you use decent passwords.. which is always the case with any device. Airports do have default passwords although it throws up errors if you attempt to use them.. Never use default passwords.. and always use decent length and complexity passwords.

If it is internal, bridged to whatever router you have (probably supplied by ISP) the Extreme is fine.. you don't need to worry at all. It has no security role in the network other than wireless which is still up to average current standard. (Not the best WPA3 which is just being released.. and I would not expect to see on older equipment).

If Extreme is your main network router Apple still should maintain security updates if required for a couple of years. Airport production ceased in April 2018.. Support should be maintained for the normal life of the device.. say 5 years.. but I believe the figure is 7 years.. that still leaves quite a bit of time.

No update has happened for a while.. but the Airport routers have a unique firmware. Based on NetBSD it is highly secure from day 1. So the router itself is secure.

NOTE carefully I said the router is secure.. people confuse router security and network security. They are not one and the same.

What is lacking is firewall. (Ignore anything you read about firewall in Apple routers.. it is simply NAT and is much like a firewall but NOT.. and is not considered security feature on any brand router).

Apple builds their clients with adequate protection. Their strategy is to build security on client rather than router.

If you run old insecure devices on an apple router it is no more secure than any other basic router.

Other brand clients may not be as secure. This is something to consider. There is nothing insecure about the Apple airport architecture.. but most modern 3rd party routers include SPI firewalls. This is useful in situations where less secure devices are used and particularly you have users/kids downloading who knows what. An SPI firewall will offer protection of the whole network.. at least to prevent the average virus or trojan calling home. Apple has never included that functionality as they believe it is security of clients where key prevention takes place.

Hope all that helps.

I use the router supplied by Frontier as the router. I then have 3 Apple airports that are connected to it to share the signal across the house in bridge mode.

Hopefully, the firmware on the Frontier router has been updated recently to bring it up to date regarding new security threats.

Only the router matters.. the airports are used purely as Access Points and totally irrelevant to security (except the wifi which is simple.. decent password)

It has been. Frontier has been pushing out updates regularly.