The past can be relevant in the right case surely? Most cases no, but occasionally.
Not in the way of, she consented with x therefore she consented with y, but if there is a distinct relevant pattern of behaviour it's different.
In this case, the other guys evidence was not that they'd had sex, but that she hadn't seemed very drunk at the time, but turned round the next day and couldn't remember what happened. If she has a history of forgetting sex afterwards, then that affects her credibility. If she has a history of not appearing drunk, but actually being so, then that can effect whether Evans had a reasonable belief she was consenting.
So, relevant, but for specific issues, not mudslinging, quite rightly.
In the same way, a woman wearing a short skirt is 98% time not relevant, but if a key witness saw a woman wearing trousers, it may become relevant for the purposes of the accuracy of what they saw.