I use Redirector in Firefox to rewrite URLs to the old Wikipedia:

  {
      "description": "Old Wikipedia",
      "exampleUrl": "https://en.wikipedia.org/wiki/Main_Page",
      "exampleResult": "https://en.wikipedia.org/wiki/Main_Page?useskin=vector",
      "error": null,
      "includePattern": "^(https?://)([a-z0-9-]*\\.)wikipedia\\.org\\/wiki\\/([^?]+)(?!=\\?useskin=vector)",
      "excludePattern": "",
      "patternDesc": "Adds ?useskin=vector to any Wikipedia page",
      "redirectUrl": "https://$2wikipedia.org/wiki/$3?useskin=vector",
      "patternType": "R",
      "processMatches": "noProcessing",
      "disabled": false,
      "grouped": false,
      "appliesTo": [
          "main_frame"
      ]
  }

I do something similar with Redirector, but a bit more involved (to also redirect mobile links to the regular pages + a few other cases):

    {
        "description": "Wikipedia Mobile -> Regular",
        "exampleUrl": "https://en.m.wikipedia.org/wiki/Hypernucleus",
        "exampleResult": "https://en.wikipedia.org/wiki/Hypernucleus?useskin=vector",
        "error": null,
        "includePattern": "https://([^.]+).m.wikipedia.org/wiki/([^#?]+)(#(.+))?",
        "excludePattern": "",
        "patternDesc": "Remove .m and add ?useskin=vector to end",
        "redirectUrl": "https://$1.wikipedia.org/wiki/$2?useskin=vector$3",
        "patternType": "R",
        "processMatches": "noProcessing",
        "disabled": false,
        "grouped": false,
        "appliesTo": [
            "main_frame"
        ]
    },
    {
        "description": "Wikipedia (Use Vector instead of Vector2022 skin)",
        "exampleUrl": "https://en.wikipedia.org/wiki/Potato",
        "exampleResult": "https://en.wikipedia.org/wiki/Potato?useskin=vector",
        "error": null,
        "includePattern": "https://([^.]+).wikipedia.org/wiki/([^#?]+)(#(.+))?",
        "excludePattern": "(https://[^.]+.wikipedia.org/wiki/Special:.+)|(https://([^.]+).wikipedia.org/wiki/[^?]+[?].*useskin=.+)",
        "patternDesc": "Add ?useskin=vector to end",
        "redirectUrl": "https://$1.wikipedia.org/wiki/$2?useskin=vector$3",
        "patternType": "R",
        "processMatches": "noProcessing",
        "disabled": false,
        "grouped": false,
        "appliesTo": [
            "main_frame"
        ]
    },
    {
        "description": "Wikipedia (Use Vector instead of Vector2022 skin) -- Query String Redirect",
        "exampleUrl": "https://en.wikipedia.org/w/index.php?title=Petabyte&redirect=no",
        "exampleResult": "https://en.wikipedia.org/w/index.php?title=Petabyte&redirect=no&useskin=vector",
        "error": null,
        "includePattern": "https://([^.]+).wikipedia.org/((?:w/index.php)|(?:wiki/([^#?]+)))[?]([^#]+)(#(.+))?",
        "excludePattern": "(https://[^.]+.wikipedia.org/wiki/Special:.+)|(https://([^.]+).wikipedia.org/wiki/[^?]+[?].*useskin=.+)",
        "patternDesc": "https://([^.]+).wikipedia.org/wiki/([^#?]+)[?].+useskin=.+",
        "redirectUrl": "https://$1.wikipedia.org/$2$3?$4&useskin=vector$5",
        "patternType": "R",
        "processMatches": "noProcessing",
        "disabled": false,
        "grouped": false,
        "appliesTo": [
            "main_frame"
        ]
    }