Apologies if I should have detailed what I meant. Let me:
There is two side of pbs+pve backup process: the pve sender and the pbs receiver side.
On the sender side the total volume may be known, especially if vzdump made a snapshot, so percentages ("progress") can be calculated.
I don't specifically know which subsystem does the actual sending but either the vzdump part or the backup-client part has the possibility to output the progress; yet I do not mean percents at all costs, bytes transferred (or even bytes processed) would belp to see what's moving and at what speeds.
On the receiver side it is true that we don't (or might not) know the total but still it would be very helpful to see the progress in bytes, like the amount of transferred bytes (/megabytes or whatever magnitude we are at) every 10 seconds, or minute or so. It would help to see how the backup goes, especially if I am already aware of the total size based on previous backups.
Both may also report the transfer speed observed, along the progress.
In case of VM there's also the rust process transferring the data which may be able to report the progress.