As the project was under license e.g. XYZ when you contributed, you licensed your code to them under the XYZ, just as much as they licensed their code to you under XYZ.
You can't demand they stop using your code, as you gave them a license to do so. The license you licensed to them under (MIT) clearly states you licensed your code in a way that allows them "to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software,"
This means they can use your MIT-licensed contributions, and bundle it in their larger plugin, and sell/license the plugin under a different license.
Technically, your contributions in their codebase is still under MIT, but since you gave them permission to commercially use it (as MIT grants), all they are legally required to do is put the MIT license at the top of the (php or whatever) code file, if the majority of that file was written by you. They aren't required to host or distribute it publicly, nor to sublicense their code under the same license. And if your code contributions were minor, they may not even have to label your code under the license, as MIT says, "The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.", and a few function additions might not be "substantial" enough to merit the requirement of demarking several lines or several functions from the greater body of work (and even if it was, merely putting them in a separate code file and importing it into their codebase would satisfy that requirement).
HOWEVER, because they also had their own code licensed under MIT, it means that any copy of the old code that they licensed under MIT is still licensed under MIT, even if they deleted the repository. You can fork the last-known MIT-licensed version (i.e. that you may have downloaded or someone else may have downloaded), and host it yourself on Github to keep it publicly available to everyone else under MIT, and there's nothing they can do about it, as long as you rename it. If it was previously called McDuffin, you could call it "McScruffin - a Free fork of McDuffin".
They'll probably shoot you a legal takedown notice to scare you, and you can then give their scare-tactic lawyers the middle-finger, replying, "I have a lawful license to distribute this code, and to sublicense it, because it was licensed to me, by your company, to 'deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so'"
You could even sell your fork and compete with them commercially.
TLDR:
- You licensed your code under MIT, so they get to use it, sublicense it to others, redistribute it, and sell it.
- They licensed their code under MIT, so you get to use it, sublicense it to others, redistribute it, and sell it.
- Neither of you are obligated to host the code or make it available for free.
- Neither of you are able to prevent the others from hosting the code or making it available for free.
- Any new code you or them write on your two diverging copies, doesn't have to be licensed under MIT.
This only applied to MIT license - other licenses vary! Some opensource licenses (GPL and LGPL if I remember right) do require users to provide copies i.e. distribute/host/"make accessible", usually just by ensuring someone somewhere in the world is hosting the code you can link to, or by emailing a copy of the code upon request.