Group errors find by type (rector, fix now, etc) rather than listing them in one big table

I'll take a look even though it's not done yet.

I quickly scanned the code and only noticed some nitpicks. As for the UI, I didn't test it because STM is down right now but, looking at the images you provided, 1) this is great (love the grouping :), 2) some of the leading info could be tweaked a little but these are nitpicks as well (see below).

Formatting:

1. +++ b/css/upgrade_status.admin.theme.css
   @@ -45,6 +45,7 @@
   +.upgrade-status-error-list tr > td.status-info, ¶
   

   Nitpick: Extra space at end of line.

2. +++ b/src/ScanResultFormatter.php
   @@ -100,9 +100,14 @@ class ScanResultFormatter {
   +            'No deprecation scanning data available. <a href="@url">Go to upgrade status form</a>.', ¶
   

   Nitpick: Capitalize "upgrade status"?

   Nitpick: Extra space at end of line.

3. +++ b/src/ScanResultFormatter.php
   @@ -267,6 +240,64 @@ class ScanResultFormatter {
   +        $this->t("Based on the Drupal deprecation version number of these, fixing them will likely make them incompatible with your current Drupal version."),
   

   Nitpick: Double quotes could be single quotes.

Wording:

1. +++ b/src/ScanResultFormatter.php
   @@ -267,6 +240,64 @@ class ScanResultFormatter {
   +        $this->t('Avoid some manual work by using <a href="@drupal-rector">drupal-rector to fix issues automatically</a> or <a href="@upgrade-rector">Upgrade Rector</a> to generate patches.', ['@drupal-rector' => 'https://www.drupal.org/project/rector', '@upgrade-rector' => 'https://www.drupal.org/project/upgrade_rector']),
   

   Nitpick: Based on the structure, it's unclear if "to generate patches" is meant to be for "Upgrade Rector" only or for both "drupal-rector" and "Update Rector" since "drupal-rector" already has a "to fix issues automatically" in the link. If it's only for "Update Rector", I would move "to generate patches" within the link.

2. +++ b/src/ScanResultFormatter.php
   @@ -267,6 +240,64 @@ class ScanResultFormatter {
   +        $this->t('It does not seem like these are covered by automation yet. <a href="@drupal-rector">Contribute to drupal-rector to provide one</a>. Fix manually in the meantime.', ['@drupal-rector' => 'https://www.drupal.org/project/rector']),
   

   Nitpick: "to provide one" seems unclear. How about "to provide coverage"?

3. +++ b/src/ScanResultFormatter.php
   @@ -267,6 +240,64 @@ class ScanResultFormatter {
   +        $this->t('Error without Drupal source version numbers including parse errors and use of APIs from dependencies.'),
   

   Nitpick: Change "Error" to "Errors" or "Error(s)" or restructure to use "These" like in some other text.

Woot! Thanks! Took a quick look and...

+++ b/src/ScanResultFormatter.php
@@ -267,6 +256,75 @@ class ScanResultFormatter {
+        // Issues to fix later need differen guidance based on whether they

Typo to fix later: "differen guidance" => "different guidance".

Looks great!