I previously had a 2018 E450 coupe that I turned in for a 2021 E450 Coupe.
I've experienced this with both cars, though more so with the newer model since I had more mileage on my previous car (pre-COVID, no WFH).

At about 4,500 miles and less than a year into having it, I got the obligatory "Service A due" for about 45 days before I took it in.
At around that time, software started to "fail". Kept telling me it couldn't download updates - "schedule an appointment".
Then, it would take forever to "boot" after starting - the phone wouldn't connect properly.
Got the service A, everything fine.

Fast forward to now. Service B starts popping up about 3 months ago (at around 8,000 miles though on my second year).
After 30 days, it gets "aggressive" - a pop up to "call now", "call later" or "dismiss" - ok, I get it, you want me to take it in for service.
At about 50 days, aside from the messages, my traffic signal assist starts failing "Traffic assist not operable". after about 15 minutes of driving.
At 65 Days, in addition to Traffic Assist, then navigation is failing "Navigation Activating" and it never does.
At 70 days, both fail, and the main menu brings up a service screen to look in the manual or go for service.
Now at around 100 days, those systems don't work, the radio is buggy (won't let me select favorites) and the latest failure is that it will not take dictation for responding to text messages.
Voice control also seems to stop understanding me.

My previous car did something similar around dates that were about 30 days overdue. Navigation would not work immediately, would have to restart the car.
With that car, I was around the right mileage, and it was faster/easier to get a service appt.

Now I get that I need to take it in for service regardless of the fact that I am 10,000 miles below the required mileage for Service B - clearly that will solve the problem. Getting an appointment lately with a loaner has been difficult at 2 different dealers.

However my question is if anyone else has experienced similar.

Once, ok. But now 2 cars behaving similarly, in similar circumstances appears on the surface at least to be some purposeful method to force you in for service.
It can't be coincidence that 2 different cars, two different model years suddenly start to flake out more and more the further past the service due dates.

Anyone else experience this?

Can't say I have ever experienced this, but I take it in for service when it's due. Keep in mind that service is due every 12 months or 10k miles whichever comes first. With a low mileage car like yours and mine, the service reminder will come on about 11 months into it, so 30 days before service is due. That should give you enough time to make an appointment. The Mercedes me connect app also recognizes if you are a low mileage driver and under Service it will tell you the exact date when you are due for service well in advance, so you have plenty of time to make an appointment even for the exact date service is due. Who knows if some of the systems start shutting down if you are way past the service due date. Haven't tried to find out.

if this is true then it is a very serious problem. What if there was an emergency and the car did not operate properly because of something Mercedes did to it?

You said yourself: Take the car in on-time. Why would anyone want to see warnings for weeks on end?

I have a low mileage vehicle (2018 <18k kms) and book the next yearly appt the day it's in the shop. Problem is avoided altogether.

Thanks for your response, however please note that I recognized in my post that taking it for service could have (possibly?) mitigated the issue;
"Now I get that I need to take it in for service regardless of the fact that I am 10,000 miles below the required mileage for Service B - clearly that will solve the problem. Getting an appointment lately with a loaner has been difficult at 2 different dealers.

However my question is if anyone else has experienced similar."

That was not the question here. The question is if anyone else experienced anything similar. I have an appointment set, and I'm sure it will be resolved, however I am concerned that a vehicle would slowly stop functioning properly if it isn't serviced precisely by the manufacturer's schedule even if it doesn't really need to be.

That was the topic. I didn't need to be told "you should have gone in for service" especially when I stated that I am aware of that.

Since my post, other things started to not function properly as well.

I will take it up with my dealer, but the question remains - has anyone else experienced similar?

Ok, something fun you can do. Make a video showing all these software issues they then reset the service (see youtube) and document if all these issues went away. There is no excuse for that kind of programming if it's true. Still do get the service though...