Formula and Levels
SAPC is the new formula that's being considered to replace the current WCAG contrast formula, which does not accurately model an illuminated display in real world ambient conditions. It also doesn't factor in colour blindness or the fact the lighter font weights are now widespread on digital mediums. The SAPC formula is not final, and no decision has been made about the minimum percentage requirements.
Large text with wider character strokes is much easier to read at low contrast. If we follow current WCAG guidelines, large text is text that is 18pt and larger, or 14pt and larger if it is bold. Considering modern phones have a definition of up to 160dpi (equivalent of 2x or 3x, iPad Mini: 162dpi, iPhone 11: 163dpi, Pixel 4a: 148dpi, 13" 1080p: 162dpi), this corresponds to 41px (regular) and 32px (bold) at the least.
The 7:1 contrast ratio is intended to address the loss of contrast that users with vision below 20/80 experience.
The 4.5:1 contrast ratio is intended to address the loss of contrast that users experience if they have low visual acuity, age-related loss of contrast sensitivity, or color deficiencies.
The 3:1 ratio is only adequate for large text (see above for definition of large text). See above for large text definition.