So far I wasn’t be able to get rounded corners in paragraphs borders, and I believe that it is at the moment impossible (unless you create a drawing image): am I wrong?
If it is so, why not plan such improvement for LibreWriter?
Thank you
am I wrong?
No you are right.
why not plan such improvement for LibreWriter?
The question is off-topic here. (And even when you file it in the proper place as an enhancement, I don’t think it would be “planned” by anyone: it will simply patiently wait for someone suddenly deciding that to be interesting to be implemented).
You have a good point about ODF compatibility. Well - but anyway, for any standard proposal to be accepted, it must have a working implementation first, so chances are … 
Presently, the only corners allowed are straight-angle corners.
You can file an enhancement request at https://bugs.documentfoundation.org (the AskLO site is manned mainy by enthusiastic users, not developers) and don’t forget to flag it as “enhancement”, not a bug. Strangely enough, there already is an enhancement request for rounded corners around tables but I didn’t find any for common borders.
However, be ready for a waiving answer such as “this is not not specified in ODF and requires a change in the standard” – which is a very lengthy process.