So unfortunately, as far as I can tell, at this time there isn't a 'catch all' option for email forwarding through Squarespace, which is very unfortunate since that was one of my favorite features using Google Domains.
However, with that being said, depending on who your web hosting provider is, there may be an alternative option. I'm currently using SiteGround for my web hosting, and using their built-in email system allows for a *@yourdomain.com catch-all email address. If I remember correctly, GoDaddy also allows this with their built-in email system, but it's been a while since I've used GoDaddy so I'm not 100% sure on that.
If you prefer to watch the YouTube video, you can watch it by clicking here.
One important thing to note: This method involves changing your domain's nameservers. So if you are hosting your website with Squarespace as well, then this method may not work for you. This only works if your domain name is with Squarespace, but your web server files/web hosting is with someone else (like SiteGround, GoDaddy, HostGator, etc.).
This is how I had to set mine up to get it working again:
(Before you begin, you'll need to add your domain to your web host, and get your Nameservers from your web host. Finding these will be slightly different depending on who your host is, so I suggest just going to Google and searching 'How to find nameservers on [your web host]')
1. Log into Squarespace Domains
2. Open Domain Settings of Domain You Want to Add Email to
3. On the right-hand side (if on desktop/laptop), under 'Advanced Settings' click 'Edit' for Nameserver Settings
4. Select 'Use Custom Nameservers'
5. Add your two nameservers from your web host (I used mine from SiteGround) and save
Just so you understand what's happening now (for those on the less tech-savvy side), essentially what you just did was tell 'the internet' anytime it needs to find settings for your domain, it will now look for those settings at your web host, rather than your domain host (in this case/my case, web host = SiteGround, while domain host = Squarespace Domains).
Now that all settings will be pulled from your web host instead of your domain host, it's time to switch over to your web host (again, in my case, SiteGround).
From your web host:
1. Go to your website/web host settings for your domain
2. Find your DNS settings (for SiteGround, they're located under 'Domain')
3. Verify your MX Records are set as your web host's built-in email system
Again, this will vary based on your specific web host. For example, with SiteGround, you can simply select the 'MX Records' tab, and it gives 3 different options: Use System Default MX Records, Use Google MX Records, or Set Custom MX Records. This is where I selected 'Use System Default' (I used to use Google when using Google Domains, but that no longer works since switching to Squarespace).
Other web hosts may not give you options and you may have to manually enter them yourself, again I suggest googling to see what the default ones are based on your web host.
Once you've either set or verified that your MX Records are using the web host built-in email system, the last step is to go into the built-in email system and re-add whatever forwarding emails you want.
With SiteGround, this was as simple as:
1. Go to 'Email' link on left-side menu
2. Select 'Forwarders' option
3. Add any forwarding emails you wish to be included, and change your catch-all address settings.
SiteGround by default has a catch-all option and is automatically set to 'Deny' - but a simple two clicks will switch it from 'Deny' to 'Allow' - and will be used after any other manual entries you make.
So I have mine set to have a few specific addresses, such as my first name, support@, noreply@, etc. Which each go to specific gmail accounts. Then I have my catch-all set to 'Allow' so anything that doesn't fall on those specific emails gets forwarded to my main one.
