Difficult to give a meaningful answer but I'm going to answer the best I can.
To me, how long someone's been in the job is irrelevant and should not be taken into account when assessing their performance. Similarly, if someone was on maternity leave but has now been back almost a year, that shouldn't be taken into account either. The job is what the job is, and either the person's done a good job during that year or they haven't.
If part of your job means you need to have current knowledge of something, then yes, it is reasonable to expect you to ensure you are up to date. If you don't want to do this during your maternity leave, that's obviously fine, but do it when you get back.
I'll use myself as an example. I don't work in one single organisation anymore but when I did a few years back I was an HR Manager. Part of my job was giving advice to managers, and if my legal/best practice knowledge wasn't up to date, I wouldn't be giving good advice, and I wouldn't be doing my job well. Obviously things change in employment law and in HR 'thinking' (as in most things) over a 2 year period, so I would anticipate that there might be things my knowledge would be out of date on. Therefore I would either keep in touch with developments during my leave, or at least make it a priority as soon as I got back to find out exactly what had happened/what law had changed/what impact this had had on my job and on the organisation. I wouldn't expect to be able to do a good job otherwise.
If however your job has changed in your absence, requiring new skills that you don't have, and everyone else had training in those skills while you were on maternity leave, and you have not been given an opportunity to have that training but have been assessed as though you have had it, then obviously that is not fair and you should challenge that.
I do agree it doesn't sound reasonable to ask you to keep in touch with clients during maternity leave - were you asked to do this before you went? I can't see what impact not doing so would have on your performance though? Presumably someone else was looking after your clients while you were away and they have now been passed back to you?
The things you mention don't actually sound like an assessment of poor performance, they sound like possible reasons that either you or they have given for poor performance.
You can see that without knowing how important the things you mention are it's difficult to advise you. I do think you should put your maternity leave to one side for a minute though, and ask yourself did you do a good job or not? Did you successfully achieve all your targets and objectives? If there is anything you haven't done as well this year, is it because you didn't update your knowledge? Did either you or your manager not realise your out of date knowledge was a problem until almost a year down the line?
You think it would be reasonable to expect you to be up to date after a 2 month absence but not 2 years. I would say the other way around. If you are only off for 2 months and something has changed, it's perfectly reasonable that you might come back and anticipate that everything is the same. If you are off 2 years, it's clear that things will have changed and if you want to do a good job, you need to make sure you are aware of the current situation.
Another point, how much could really have changed in 2 years that you couldn't have updated yourself with in a very short time?
Apologies if that's not what you want to hear, and also if I've misunderstood, but I've answered based on my understanding of what you are saying.