Can I email users who signed up?

Not a lawyer, but familiar with GDPR

No, this is not legal under GDPR and most privacy & spam-mail regulations.

Your users did not explicitly consent to receive updates regarding your product. These updates would be considered advertising, which, again, they did not sign up to receive.

Broadly speaking, you can only send your users an eMail if:

• They explicitly agreed to receive them
• The eMail is vitaly important to fullfill your contract with your user
• The user can reasonably expect to receive such eMails as part of your service

So if there's a service disruption / change to TOS, change in pricing, etc. it would be reasonable to assume that your users need to know about it in order for you to fullfill your contract / your TOS with them / because your users can reasonably expect to receive such eMails when signing up for your services.

Just notifying your users regarding big updates is advertising and not covered under the latter two reasons and can only be done if you acquire consent from your users under the first point (explicit agreement).

It is not legal for EU citizens (they have to opt-in), not sure about the rest of the world. Aside from legality, as a user i find it annoying and will add your to the spam filter.

This is not legal advice and I am not a legal expert.

Mailchimp allows you to import your lists of people and also gives them an opt-out option. If your emails are going through Mailchimp I think they have done the legal checking about what emails can be sent, opt-outs, etc. Also it is free for a certain amount of subscribers.