MDL-50660 was created to get rid of some .IE6 prefixed selectors, we started a discussion there and in https://moodle.org/mod/forum/discuss.php?d=315571 about whether to remove them or not.

Dan commented (https://tracker.moodle.org/browse/MDL-50660?focusedCommentId=360371&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-360371) that it was decided (probably in MDL-41973) that we would not intentionally break support for older browsers even if we don't officially support them. I understand that cases like OU and and IE8 can fit into this case, but IE6 seems to reach a new level of "user experience" as moodle has evolved a lot since it was supported. According to Gareth (https://tracker.moodle.org/browse/MDL-50660?focusedCommentId=360412&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-360412) moodle seems unusable in IE6.

We could test master in IE6 a bit more to be sure that the user experience, but IMO it does not make sense to keep these IE6-specific selectors there if basic moodle functionalities are not available.

This policy should not be restricted to IE6, if we have IE7 selectors and IE7 is also unusable I would vote for also getting rid of them, same with other specific unsupported browsers selectors in core.