I have created a office 365 group and for some reason i have deleted it. After 10 days when I try to create office group with same name, group alias is available but unfortunately site url is not good. For example if I try to create a office group with HR then site url is coming as https://<domain>.sharepoint.com/HR20 or https://<domain>.sharepoint.com/HR2 or https://<domain>.sharepoint.com/HR48 etc.
There is no possibility to create HR office group by having url as https://<domain>.sharepoint.com/HR?
When you delete an Office 365 Group, the associated SharePoint site is sent to a recycle bin. You can see the list of Deleted Sites (aka sites in recycle bin) by going to the newer/preview SharePoint Admin Center. In order to be able to recreate a new SharePoint site with the same name you used before you need to delete the old site from the recycle bin. To do so you will need to use PowerShell. Look for the "Remove-SPODeletedSite" command on Microsoft Docs for more information on how to remove deleted sites from the recycle bin. As long as your old site is in the recycle bin, you won't be able to create a new site with the same name. I also recommend running the Get-SPODeletedSite command in PowerShell to see all of the sites in your recycle bin, it may have more listed there than are showing up in the Admin Dashboard. My recycle bin somehow had two instances of the same-named site. I had to remove both of them before I could create a new site with the same name.
I wouldn't say the site recycle bin is hidden for modern sites, but it is not available from the current SharePoint Admin Center. To see a list of Active or Deleted (in recycle bin) modern sites, you need to go to the Preview version of the SharePoint Admin Center or use PowerShell. You get to the Preview admin center by going to the current SharePoint Admin Center, then clicking the "Try the Preview" button located toward the top-right section of the page. I would recommend using PowerShell though, since you will need to use it to remove sites from your recycle bin anyway.