All depends. Do you know if you have a clean-out elsewhere? Generally they're very close to where the main drain leaves the house, not the middle of a floor. Is there a smaller drain nearby? If so you've got an abandoned toilet & shower. If you have no idea, I'd err on the side of caution and leave some access to this. I've seen cleanouts go straight down, Tee, Y, or take an immediate 45º. Check your utility room for a big drain with a Y shaped pipe with a threaded cap on it. Honestly I doubt they'd put the only cleanout in the middle of a floor.

Plumber here - I have my doubts that is a cleanout, but rather instead an abandoned toilet without a flange. You can confirm by pulling off that knockout cap and look down, if it turns a corner a few inches down, its a toilet. If so - get an inside pipe cutter, and inside test plug. Cut the pipe off flush with the concrete, put the test plug in a couple inches down, use a chipping hammer to level the lip around the pipe, then cap with concrete.

In my case they found an already weakened section and then grew enough to cause a blockage, leading to me discovering the problem(s)

They have a tendency to find their way into sewer drains and cause a lot of damage.

OP do you have Willow trees? If so you'll want to get an inspection done.