Comment # 1 on bug 976985 from Reinhard Max

Indeed, something in tclConfig.sh must have changed from 8.5 to 8.6 that broke
the linking of the plugin. But even after explicitly linking it to libtcl8.6
(which contains the Tcl_Init symbol), it fails to start without printing
anything to stderr.

There has been some effort to modernize the Tcl plugin, but integrating that
would take more time than I currently have:
https://fossil.e-lehmann.de/tclplugin/home

Just out of curiosity: what are you planning to do with the Tcl plugin?