Variable SLACK_TEMPLATE_LIMITS`Const`

SLACK_TEMPLATE_LIMITS: {
    altText: 2000;
    blocks: 50;
    buttons: 5;
    buttonText: 75;
    contextElements: 10;
    fallbackText: 3000;
    fields: 10;
    fieldText: 2000;
    headerText: 150;
    sectionText: 3000;
    url: 3000;
} = ...

Slack's documented ceilings, enforced at save (§5.1).