Is it necessary to quarantine older, healthy fish that weren’t originally quarantined?

[Martingale]

I have a couple fish that have been in a tank without additions of any kind (invert, fish, or coral) for over a year now, but they were originally not quarantined.

I was looking to start doing online orders and quarantining purchases, but I was wondering if it is necessary to quarantine/run-fallow my current stock?

 

[gentlefish]

There would be no benefit in removing the fish for quarantine or treatment now. No fishkiller organism in the tank.
And yes I very strongly recommend quarantaining EVERYTHING new going forward as you had already planned!