Is Macintosh-data partition deletable/undeletable..?

Please see: About the read-only system volume in macOS Catalina - Apple Support

Hope it helps.

Jay__Sam wrote:

Why every time I try to make an Installation of MacOs Catalina it happens to appear the Macintosh-data partition to be there after the installation and if I try to go back to the recovery model and delete the Macintosh-data partition the system wont boot..?

If you correctly install the macOS you would expect to see Macintosh HD and Macintosh HD - Data.

From the Terminal you can see the drive structure here:

diskutil list internal

for example only—Catalina

MacBook-Pro ~ % diskutil list internal

/dev/disk0 (internal, physical):

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *1.0 TB disk0

1: EFI EFI 314.6 MB disk0s1

2: Apple_APFS Container disk1 1.0 TB disk0s2

/dev/disk1 (synthesized):

#: TYPE NAME SIZE IDENTIFIER

0: APFS Container Scheme - +1.0 TB disk1

Physical Store disk0s2

1: APFS Volume Macintosh HD 11.2 GB disk1s1

2: APFS Volume Macintosh HD - Data 273.7 GB disk1s2

3: APFS Volume Preboot 80.8 MB disk1s3

4: APFS Volume Recovery 529.0 MB disk1s4

5: APFS Volume VM 3.2 GB disk1s5

Under the the new APFS all Volumes share space within the Container without penalty.

https://support.apple.com/guide/disk-utility/add-erase-or-delete-apfs-volumes-dskua9e6a110/mac

If it keeps appearing after every time you install, does it not occur to you that it might be part of the OS.

In Catalina and later, the OS is split into a system volume and a data volume. Your data is stored on the data volume.

Appear where? In the Finder? In Disk Utility.

When you install Catalina, two separate volumes are created - say, "Macintosh HD" and "Macintosh HD - Data".

That is normal and if you delete the Data volume you are deleting all the user account files.