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: