Boot from USB is supported. The VM needs to be EFI as virtual firmware.
You can boot from a bootable Linux USB or bootable Windows 10 using Workstation VM with EFI virtual firmware.
With Workstation 14.x you can choose the virtual firmware when you create a custom VM. Otherwise you could also add the line to the vmx configuration prior to installing the guest OS.
You can then "Power on to Firmware". Connect the Bootable USB from the Devices Menu to the VM; enter the EFI setup and go back out to the EFI main menu and the "EFI USB" should become available as a choice for booting up.
Mi riferivo al boot USB Legacy
I don't quite understand what you meant by legacy USB boot.
But if you don't want to use EFI virtual firmware and still want to use virtual BIOS, you can try to use a tool such as Plop boot kit. Even with physical machines with BIOS, depending on SMBIOS version (and thus the age of the physical machine), you may not be able to boot from USB flash drive.
The path of least resistance for USB flash drive boot with VMware Workstation is still to use a VM with EFI virtual firmware as I had described earlier.
Sono d'accordo che su vecchie macchine con vecchi BIOS non c'era il supporto per l'avvio da USB, ma si tratta di macchine molto vecchie. Lo stesso discorso vale anche per le prime versioni di VMWARE WORKSTATION. Mentre non capisco perché con le versioni più recenti, comunque prima dell'introduzione di EFI, non sia stato implementato l'avvio da USB nel Virtual BIOS.
You and I can speculate the reasons but I don't think we will know why virtual BIOS in VMware does not support USB boot; unless some current/former VMware employee(s) who has inside knowledge will come to share the story (highly unlikely).
Sometimes, features of a software is not just dictated by technology but also by business/contract reasons.
For one thing, it would look that VMware would have more control/flexibility in adding features to Virtual UEFI versus virtual BIOS. If you look at msinfo32 in a Windows VM, you will see "Phoenix Technologies" for a "BIOS Version/Date" with virtual BIOS while one with virtual UEFI shows "VMware Inc"; which suggests that virtual BIOS was licensed from Phoenix (possibly with modification restrictions or without source code) while virtual EFI was made by VMware. So to add USB boot support to virtual BIOS might have meant Phoenix Technologies modifying the virtual BIOS and/or VMware possibly having to pay additional fees for a Phoenix BIOS (or another BIOS vendor) with such a feature. This is also an example that you and I can speculate the many possible reasons but will never ever know...
Anyway, Intel has announced in a presentation in 2017 that BIOS will no longer be supported by 2020 but there is no clear roadmap of how this will happen. Even before this announcement, some add-in graphics cards don't even work with physical machines with BIOS (ie. the machine won't boot with the card installed) and works exclusively only with UEFI. It would be best any new VM(s) you create use virtual EFI or seriously consider converting existing VMs with BIOS to EFI.