Yes, you can add random extensions (or write your own) using developer mode. A common use case for this is to install the Bypass Paywalls extension from outside the official extension store, which will allow you to read news stories from a bunch of sites without a subscription.
But sideloading isn’t the issue here. Once they move to a new extension system (“manifest v3”), they will be limiting the browser features that can be controlled by extensions. The old subsystems that used to allow explicit blocking by extensions will be replaced by a list of blocking suggestions that extensions can provide, but the browser can choose to honor or ignore them as it wishes.
So even if you sideload your own extension, you won’t have access to the old-style explicit blocking anymore. Older extensions (sideloaded or not) can explicitly make the browser block X, Y, and Z. Under the new system, it’ll have to ask the browser “pretty please block X, Y, and Z”, but the browser might just decide “nah, I’ll only block X and Y because Z makes us too much money, too bad”.
The uBlock Origin team has a FAQ on the transition (though it’s more developer-focused than for end-users). TLDR ad blocking will be weaker with manifest v3, but still there. The ads you want to block may or may not still be blocked… hard to say without trying it yourself on the sites you normally visit. But the blocking capability is permanently neutered, and once Chrome starts going down this path, future versions may be even more neutered, and there’s nothing that extensions can do about it.
Fortunately, as long as the rendering engine underlying Chrome (Chromium, or Blink one level lower) remains open-source, alternative Chromium-based derivatives like Brave can continue to offer the older, more powerful blocking functionality. And of course completely different browsers like Firefox (which has its own problems) or the newer, experimental Ladybug can continue do their own things – they just have to live with incompatibilities between them and Blink/Webkit, which most real users and devices use. There’s also the Orion browser (Webkit-based so more compatible, but only for Mac & iOS); it’s made by Kagi, that upstart subscription-only Google Search competitor, meaning they are a direct competitor to Google and more likely to take your side in the ad wars. These other browsers may offer fewer ads, but they’ll sometimes have their own minor incompatibilities on certain sites.