As mentioned in #4718, this module has been rotting in the plugins package for several years and should be removed.
It was added in d499a48 back in 2014 and was used by the following plugins

• connectcast (removed plugin in 6571b28)
• euronews (updated in 777a96f)
• sportal (updated in a91e134)
• tv5monde (removed plugin in 3c85180, reimplemented plugin without common_jwplayer in b3efc27)

$ git log --format=reference -S common_jwplayer -- src/
3c851805 (plugins.tv5monde: remove plugin, 2021-10-07)
57eea0fb (plugins.sportal: Removed old RTMP streams, use HLS, 2018-08-04)
e50e60b2 (Add support for TV5Monde, 2017-06-02)
8329f701 (plugins.sportal: added support for the live stream on sportal.bg, 2017-01-10)
777a96f9 (plugins.euronew: update to support the new site (#141), 2016-11-08)
30fb8623 (Fix connectcast.tv livestreaming, 2015-02-01)
dca69a2a (plugins.euronews: Update for service changes., 2014-09-03)
d499a488 (plugins.connectcast: Move JWPlayer playlist parsing to support plugin., 2014-09-03)

The docs's plugins extension and the generic plugins tests still have checks for modules inside the plugins package whose file name starts with common_, but those checks don't necessarily have to be removed with the removal of the (last) common_ module:

• streamlink/docs/ext_plugins.py

  Lines 184 to 190 in 156b660

  	def __init__(self):
  	plugins_path = Path(streamlink_plugins.__path__[0])
  	self.plugins = [
  	(pname, plugins_path / f"{pname}.py")
  	for finder, pname, ispkg in pkgutil.iter_modules([str(plugins_path)])
  	if not pname.startswith("common_") and pname not in self.protocol_plugins
  	]

• streamlink/tests/test_plugins.py

  Lines 27 to 31 in 156b660

  	plugins = [
  	pname
  	for finder, pname, ispkg in pkgutil.iter_modules([plugins_path])
  	if not pname.startswith("common_")
  	]