Is it wise to let the PO know that I do not know much of the product API's because of many factors and I am in the 8th month of my employment?
You've already established something of a reputation as being a bit below the company's (not unreasonable) expectations for someone hired as a senior developer. All this would do is cement that reputation further.
An impeded onboarding is unfortunate (and it sounds as though the company has taken responsibility for that aspect) but how long are you planning on riding that particular issue?
8 months is a heck of a long time - I'd expect someone in a senior developer role to be able to get a decent handle on the code base in a self-led manner in a far shorter time-frame than 8 months. To be blunt I've seen a good amount of juniors get up to a decent, productive knowledge level in less time than that.
Look I get it, the last 6-7 months have been hard on us all with COVID and whatnot, and I'm sure that's been a factor in the company cutting you a certain amount of slack. But that slack is going to be finite - it has to be finite, and you've got a choice as to how you use however much of that good will time that remains to you.
You're on your probation period (which has already been extended once) - and at the end of that period there's probably going to be some kind of review to discuss your progress. So what do you want that review conversation to be?
You can have either:
- A conversation about you not knowing the APIs yet where you do your best to prepare some excuses about how it was the lack of onboarding etc. Maybe they give you another extension, maybe they just cut their losses.
- A conversation about you taking the initiative to learn the APIs etc yourself and how you've overcome the less-than-ideal onboarding and are now productive in the role they hired you for.
Of course the one of these that results in a better chance of keeping your job is significantly harder to do than the other, but it's not a co-incidence that the one that involves more work is the one that actually reflects what a "senior" developer would be expected to be doing. But it's not all doom and gloom, after all that's why they get the big bucks
At this point it's really your call (assuming time and good will haven't already run out of course), and the only person who can get yourself to that second version of the conversation is you.
Maybe it's too late to turn it around, maybe you think it's too big of an ask given that you are already on shaky ground in this role and you think you'd be better off trying afresh elsewhere.
If you want to do that however then you need to be honest with yourself about whether you are ever prepared to take that second option - because in a lot of senior roles on-boarding is relatively minimal, you're expected to teach yourself a lot of what you need to know while getting productive in as short a time as possible.
So if you aren't willing or able to do that, and you need a bit more of a hand-holding management style then you need to realign your career accordingly and look at more junior positions. There's no shame in that, and there's nothing to say that you couldn't earn a solid senior position at somewhere by being promoted internally/getting raises etc. once you already know their environment inside and out.