I don't know why, but I always get confused if the far right is the current hour or the far left is the current hour. A mouse-over, tool tip or guide that just says "current hour" or "24 hours ago" would help clarify at a glance.

Give a name to phone numbers! otherwise it's hard to know who they belong to.

I believe the effort to turn you application into an webapp is pretty low (just include the manifest.json into the project root), and that would enable the users to install the website and use as an app.
It would also open some doors for future development:
- Send push notifications from web
- Offline access (yeah, this is kinda weird for you product)
- Improved caching
- Auto-updates straight from web (this is in comparison to native apps)

I would like to have checks which fail when something is there which shouldn't be there. For example a http endpoint which should only be available through https.

I would like to set which Slack Alert Channels (but this idea applies to all notification methods) should be default on or default off.

I have a couple of channels I need just for specific high priority notifications. I don't want them accidentally switched on for every updown.io entry that is added

I like ordering checks by status having the "down" first.
It would be nice, in this view, to have the disabled checks at the bottom of the list.

Bulk option: change the apdex time level, e.g., quickly switch between .5 seconds and 1 second to see difference.

Add screenshots to the reports, so we don't have to setup that with another provider seperatly

I'd like to be able to enter multiple email addresses for the weekly report. For example, certain department managers would like to receive a weekly report of the checks, but not be disturbed every time a check goes up or down.

On the dashboard it would be nice to see the timing to the first response

Increase the frequency choice to 1 day

To save time on bookkeeping, add UBL invoices.

We would love to be able to post alerts to Google Chat using the incoming webhooks (cf. https://developers.google.com/hangouts/chat/how-tos/webhooks). A similar integration to the one already existing for Slack would be wonderful!

Right now I have about 20 subdomains across 3 top-level domains.

The current "domain-suffix-oblivious" lexical ordering is kind of a mess.

I really wish all my "hexstreamsoft.com" subdomains were grouped together, and same thing for my "hexstream.net" and "hexstream.xyz" subdomains separately. Right now those are all "randomly" jumbled up together based on the lexical order of their lowest subdomain components.

At the moment a revoked certificate will show an error on browsers, but will show as "up" by updown.io

I was affected by the following, and didn't receive any notification when the site was inaccessible
https://arstechnica.com/information-technology/2018/03/23000-https-certificates-axed-after-ceo-e-mails-private-keys/

It would be nice to be able to choose which notification recipient (email, sms, ...) to activate/deactivate when creating a check through the API.
For now all recipients are activated by default.

It would be nice to have either page specific or account wide custom stylesheet applied to the public status pages. Specifically, I'd use it to draw more attention to the translation dropdown and to eliminate some whitespace.

Send status page url in webhook data

Provide integration for https://www.pagerduty.com/

Instead of making the Slack webhook integration via the API, just allow users to put in a Slack URL. That way any other service which can emulate the Slack webhook schema can use the webhook feature.

As an example Discord has webhooks and if you add "/slack" to the end of a Discord webhook URL then it will parse the data as Slack does.

Currently I cannot use Updown with Discord. I also suspect that many other programs have an option to parse data like Slack so you could knock out a few webhook integrations in one simple step.