Simply leaving a lane isn't punishable as long as you are still actively trying to win.
Remember that support singed's ban was for things other than simply roaming, and that his ban was overturned.
Remember that
does exist, hence "support must be restrained to bot lane" is not an actual rule.
**keep in mind though, that leaving lane will likely get you reported by at least someone on your team, and it's likely to be your toxic adc. This will flag that particular game for potential review, but if it's not a regular thing and if you weren't a toxic teammate yourself you have nothing to worry about.