SharePoint Online: Web Designer Galleries (Site Columns, Content Types, Web Parts, List Templates, etc) Missing in Site Settings

Problem: Web Designer Galleries section with below links missing in SharePoint Online Site Settings page:

• Site Columns
• Content Types
• Web Parts
• List Templates
• Master Pages
• Themes
• Solutions
• Composed Look

Also, under the "Site Collection Administration" section: HTML Field Security, SharePoint Designer Settings links missing!
Solution: Enable Custom Script!
To enable custom script in SharePoint Online, do the following:

• Sign in to Your SharePoint Online Admin Center site: https://YourDomain-Admin.SharePoint.com
• Click on Settings from the left navigation >> Scroll down to "Custom Script" section
• Under the Custom Script section, Set "Allow users to run custom script on personal site" and "Allow users to run custom script on self-service created sites" options to enable custom script SharePoint online.
• Click on "OK" to save your changes.

However, this change may take up to 24 hours to reflect. To enable custom script on a particular site collection immediately, use this PowerShell script through SharePoint Online Management Shell.

PowerShell to Enable Custom Script in SharePoint Online Site Collection:

#Config Parameters
$AdminSiteURL="https://crescent-admin.sharepoint.com"
$SiteURL="https://crescent.sharepoint.com"
   
#Get Credentials to connect
$Cred = Get-Credential
   
#Connect to SharePoint Online Tenant Admin
Connect-SPOService -URL $AdminSiteURL -Credential $Cred
   
#Disable DenyAddAndCustomizePages Flag
Set-SPOSite $SiteURL -DenyAddAndCustomizePages $False

You can also use PnP PowerShell to disable or enable "DenyAddandCustomizePages"

#Site Variables
$AdminSiteURL = "https://crescent-Admin.sharepoint.com"
$SiteURL = "https://crescent.sharepoint.com/sites/credit"

#Connect to Admin Center
Connect-PnPOnline -Url $AdminSiteURL -Credentials (Get-Credential)
$Context = Get-PnPContext

#Get the Site collection
$Site=Get-PnPTenantSite -URL $SiteURL

#Disable Deny Add-Customize Pages
$Site.DenyAddAndCustomizePages = "Disabled" #or Enabled
$Site.Update()
$Context.ExecuteQuery()

Here is the side by side comparison of DenyAddAndCustomizePages flag Disabled and the Enabled: