1. why they removed the latest firmware (and if it is causing bugs or other problems)
The latest firmware was removed because it could potenially brick DS1000Z models with an old boot loader. This has been discussed on this forum, you need to search the relevant threads. This latest release can still be downloaded if you know the URL (also posted somewhere here), since they only removed the download link.
2. if it is save to upgrade anyway (maybe somebody could provide the latest firmware)
While some users have upgraded before the firmware with removed and claim to have no issues. One of the main benefits of the new firmware version is that it allows a full screen XY render.
3. or if I should downgrade (if my new DS1054Z already had 00.04.04 installed on it) because of bugs/problems
As far as I know, you cannot downgrade the Rigol firmware! Once you upgrade, there is no turning back.
Regarding your previous questions and common questions:
a) I received my Rigol DS1054Z last friday. It came with firmware version 00.04.03.SP2. The board version is 0.1.4. It is with older board versions that the latest firmware had issues with SOME devices.
b) I believe that the corrected firmware will be published soon enough, hence there is no need to take a risk and upgrade with the current removed version.
c) I don't think you will "experience" the "jitter bug", even if your device comes with a firmware version that has this "bug". Most bugs mentioned in this forum are bugs on a much augmented scale, using specific examples to highlight the problem. With regular/normal use I don't think you will suffer from this bug.
d) I successfully enabled all options without a problem. BUT: don't use Windows 10 internal telnet client: it does not work properly on port 5555 with the Rigol! I used Putty and even so, I did not receive any message upon connection on port 5555. I only got something on screen after sending a command. This was not mentioned anywhere, so I add it here for newbies. Just copy&paste the command into the Putty session window and it will be accepted. I wasted like 15 minutes figuring why I didn't get any prompt upon connection...
e) Coming from a basic 2 channel 20MHz CRT scope, I am overwhelmed about the many functions and options this scope offers. I mean there are literally HUNDREDS of options/settings/functions! My old CRT scope has like 10-20 buttons and that's it... So the learning curve is huge on this DS1054Z, which is proof that I did well in getting this scope, even if it was just to learn how to operate a modern DSO. So while you are waiting for that new firmware, use the time to get to know your new scope, when it arrives.
f) 4 channels? Initially I questioned the value of it, but now I got it. Imagine testing a device communicating over i2c and there is a problem. You need two channels to monitor i2c (clock and data) and then for instance a third channel to monitor the produced output signal. Bingo: three channels!
Regards,
Vitor