Some time ago, I actually had a look at this issue. Further (rather technical) details can be found here (https://github.com/taminob/shortcuts/issues/1). In short: It is possible to disable it (with probably some drawbacks), but then again you might not be able to assign new actions to that shortcut. Not sure how this would affect special characters. However, at least for me, alt+shift+[1-9] does not switch tabs so I also might misunderstand the issue.