Privacy Browser Android

You are right in that this, apparently simple feature is quickly turned into a complicated mess when one starts thinking about all the possible URL modifications that the end-user might try to apply. So, I suggest thinking about the simple first steps to begin with.

Removing URL part: basically when that string is located in the URL, it is removed. It is up for the end user to be careful deciding what has to be removed.
Replacing URL part: whenever a certain string appears, it is replaced with another string.
Adding URL part: since we already covered "replacing", adding means basically at the end of the URL add some other string. It could also be done at the beginning of the URL, but maybe start with the end for now.

The same code used for removing Facebook tracking could be used to remove ANY part of the URL string and it shouldn't give any trouble (like I said, it's an advanced feature that leave up to the END USER the responsability of what he is deleting and wether or not the URL will load after).
All this would be a simple extra menu in the "Domain settings" in which you could have "add string" "remove string" and "replace string" options. For a start maybe have only ONE option available for a domain, at the same time.

Of course it's easier to say that do, but I hope this at least shows a clearer path of logic as to how this feature should work (for now at least, of course in the long run one could expect to use wildcards and such, but starting small and simple should help a lot).

This is a general implementation of the specific feature described at https://redmine.stoutner.com/issues/505. Likely one interface will be able to handle both.

There is also some discussion of this topic at https://redmine.stoutner.com/issues/844.

Allow user to use split, replace/replaceAll, slice, splice etc methods. Add this option to Domain list.

I am not sure I understand what you are requesting. However, I should make two points.

1. This feature request is about URL modification, not Domain Settings. If you are referring to Domain Settings, you should create a new feature request.
2. For performance reasons, I am unlikely to implement anything that would require processing regular expressions. Some of the words you are using make me think that perhaps you are requesting something that would fall into that category.

https://en.wikipedia.org/wiki/Regular_expression

Is this feature request for Link Redirection ?

Yes, that is one of the things this will be able to do.

The most basic use case for me, is to redirect YouTube videos to piped. An URL typically youtube.com/watch?v=<video-id> should redirect to piped.video/watch?v=<video-id>. Same goes to other sites. But this is the basic usecase I'm familiar with.

This feature will be able to do that.

I've recently started relying on libreddit for reddit too. Everything from the reddit URL is same, except the base domain part.

For example:
youtube.com > piped.video
reddit.com > safereddit.com

by keeping the root uri same.

That's exactly the type of modification this feature will allow.

Maybe introduce this into Domain settings itself. I think it would be a good idea. Just add a textbox for redirection link.

This needs to be separate from domain settings because it can modify the domain, which could change which domain setting would apply. Doing this in domain settings could easily result in endless loop problems.

Rather, this will be a separate setting that will be applied before domain settings.

We can actually call this as Domain Redirection too. For instance, you can place this at the top of the Domain settings. If set up, grey out all the rest of the configuration for that domain.

That wouldn't accomplish all this feature request needs to do. I think it is best to leave URL modification as a separate interface.

It makes sense for a full fledged implementation of this. But just reminding you that this'll become a very jumbled up situation.
Modify in the sense how ? The author is asking some crazy modification like adding &listen=1&autoplay=1 at the end. This is complicated because it should only be applied if watch? exists. And it's just one example.

I don't think this'll be helpful to anyone and too niche to even begin with. Setting aside #844 to be a very simple FR that tries to achieve a basic thing which is also a very important function to a lot.