You now need to change the makedepends to

makedepends=("nginx-mainline-src={your nginx version from nginx -V}" {makedepends from https://gitlab.archlinux.org/archlinux/packaging/packages/nginx-mainline/-/blob/main/PKGBUILD })

so currently

makedepends=("nginx-mainline-src=$pkgver" mercurial pcre2 zlib openssl gd geoip mailcap libxcrypt libxslt rsync)

This package has been out-of-date for quite a while so far and it refuses to be installed with newer version of nginx. To get it compiled with your nginx installation, a quick workaround is to change line 12 of the PKGBUILD file to

makedepends=("nginx-mainline-src={your nginx version from nginx -V}")

Repentinus: Could you please update the package for nginx 1.26? Here's a patch that also makes the script consistent with the current version of the nginx-mod-http-xslt-filter package:

diff --git a/PKGBUILD b/PKGBUILD
index 6f7160b..d7f923d 100644
--- a/PKGBUILD
+++ b/PKGBUILD
@@ -5,33 +5,31 @@
 # Based on nginx-mod-http-xslt-filter AUR package.

 pkgname='nginx-mainline-mod-http-xslt-filter'
-#pkgver=$(pacman -Si nginx-mainline-src | sed -nE 's/^Version *: ([[:alnum:]._]+).*$/\1/p')
-pkgver=1.19.6
-#pkgrel=$(pacman -Si nginx-mainline-src | sed -nE 's/^Version *: [^-]+-(.*)$/\1/p')
-pkgrel=2
-pkgdesc="HTTP XSLT module for the Nginx mainline web server"
+pkgver=1.21.6
+pkgrel=1
+pkgdesc='Nginx mainline module for transforming XML responses using XSLT stylesheets'
 arch=('x86_64')
 url='https://nginx.org/en/docs/http/ngx_http_xslt_module.html'
 license=('custom:BSD-2-Clause')
 depends=('nginx-mainline' 'libxslt')
-makedepends=('nginx-mainline-src')
+makedepends=("nginx-mainline-src=$pkgver")

 prepare() {
   cp -r /usr/src/nginx/ ./
 }

 build() {
-  cd nginx
+  cd 'nginx'
   _options=$(nginx -V |&
              sed -nE 's/^configure arguments: ([^\n]*)$/\1/p' |
              sed -nE 's/([^'"'"' \t\n]+('"'"'([^'"'"'\]|\\'"'"'?)*'"'"'|"([^"\\]|\\"?)*")?) ?/\1\n/gp')
-  IFS=$'\n' xargs ./configure --with-http_xslt_module=dynamic <<< ${_options[@]}
+  xargs ./configure --with-http_xslt_module=dynamic <<< ${_options}
   make modules
 }

 package() {
-  cd "nginx/objs"
-  for f in *.so; do install -Dm644 "$f" "$pkgdir/usr/lib/nginx/modules/$f"; done
+  cd 'nginx/objs'
+  install -Dm644 -t "$pkgdir/usr/lib/nginx/modules/$f" *.so
   install -dm755 "$pkgdir/usr/share/licenses/"
-  ln -s /usr/share/licenses/nginx/ "$pkgdir/usr/share/licenses/$pkgname"
+  ln -s '/usr/share/licenses/nginx/' "$pkgdir/usr/share/licenses/$pkgname"
 }