That's how politics works. If a conservative MP had done something similar in 2011, you can bet every penny you have it would have been brought up too.
There is a good way to prevent stuff like this though - don't be horrible!
'That's how politics work' - you mean using stories from 6 years ago to try and discredit a political party. Labour has been too nice to May as there are plenty of skeletons in her cupboard to do with her time as Home Secretary from 2010.
If only they would bring things up more often from 2011, to discredit May and the Tories. It wouldn't be too difficult. But if they did, the anti Corbyn brigade would be shouting and screaming that he is 'playing dirty'.
The Tories are desperate and as usual will use all the dirty tricks in the book including lies, slurs and innuendos. As they have done every since Corbyn stood to lead the Labour Party. You were quite happy to peddle a story from 2011 to attempt to discredit Labour. But heh ho 'that's how politics work'.
Edit.Maybe Johnson should practice the 'don' t be horrible'. What did he say to that taxi driver? I just can't remember.