hmm the required b/c code has been introduced in 3.9.11 (by #25225 in response to: #25204):
https://github.com/joomla/joomla-cms/blob/staging/libraries/src/Router/Route.php#L72-L93

based on that the signature you proposed here it should still work than expected as

echo (int) true; -> 1
echo (int) 1; -> 1
echo (int) '1'; -> 1

please double check the signature you use and the joomla version let us know which it is so we replicate the issue.

hello, i tested on 2 different servers now :

echo JRoute::_('index.php', false, true)."\n";  //returns http:
echo JRoute::_('index.php', false, false)."\n"; //returns relative
echo JRoute::_('index.php', false, 1)."\n"; //return https:
echo JRoute::_('index.php', false, '1')."\n"; //returns https:
echo JRoute::_('index.php', false, JROUTE::TLS_DISABLE)."\n"; debug_zval_dump(JROUTE::TLS_DISABLE); //returns http: 
echo JRoute::_('index.php', false, JROUTE::TLS_FORCE)."\n"; debug_zval_dump(JROUTE::TLS_FORCE);  //returns https:
echo JRoute::_('index.php', false, JROUTE::TLS_IGNORE)."\n"; debug_zval_dump(JROUTE::TLS_IGNORE);  //returns relative

stan

and for the true boolean I always get non-https reply

this is on PHP7.3 , 7.2, 7.1, 7.0 and 5.6

stan

i just debugged this and used:

if ($tls == -1)
			{
				if ($tls === true) {
					die('x'); 
				}
				
				$tls = self::TLS_DISABLE;
			}

if $tls === true it also evaluates to $tls == -1

i did not know about this, but this is what's causing the problem

stan

so it gets disabled before it reaches conversion to int

Strange but Confirmed here is a patch: #26089

reopened @franz-wohlkoenig ?

ups, sorry.