We need to follow this text from ARIA 1.1:
If aria-readonly is set on an element with role grid, user agents must propagate the value to all gridcell elements owned by the grid and expose the value in the accessibility API. An author may override the propagated value of aria-readonly for an individual gridcell element.

In a grid that provides cell content editing functions, if the content of a focusable gridcell element is not editable, authors may set aria-readonly to true on the gridcell element. However, the value of aria-readonly, whether specified for a grid or individual cells, only indicates whether the content contained in cells is editable. It does not represent availability of functions for navigating or manipulating the grid itself.

An undefined value for aria-readonly, i.e., aria-readonly is not specified, does not imply that a grid or a gridcell contains editable content. For example, if a grid presents a collection of elements that are not editable, such as a collection of link elements representing dates in a datepicker, it is not necessary for the author to specify a value for aria-readonly.
 

We need to follow this text from ARIA 1.1:
If aria-readonly is set on an element with role grid, user agents must propagate the value to all gridcell elements owned by the grid and expose the value in the accessibility API. An author may override the propagated value of aria-readonly for an individual gridcell element.

In a grid that provides cell content editing functions, if the content of a focusable gridcell element is not editable, authors may set aria-readonly to true on the gridcell element. However, the value of aria-readonly, whether specified for a grid or individual cells, only indicates whether the content contained in cells is editable. It does not represent availability of functions for navigating or manipulating the grid itself.

An undefined value for aria-readonly, i.e., aria-readonly is not specified, does not imply that a grid or a gridcell contains editable content. For example, if a grid presents a collection of elements that are not editable, such as a collection of link elements representing dates in a datepicker, it is not necessary for the author to specify a value for aria-readonly.

Also:
While the columnheader role can be used in both interactive grids and non-interactive tables, the use of aria-readonly and aria-required is only applicable to interactive elements. Therefore, authors should not use aria-required or aria-readonly in a columnheader that descends from a table, and user agents should not expose either property to assistive technologies unless the columnheader descends from a grid.
 

We need to follow this text from ARIA 1.1 (for both grid and treegrid):
If aria-readonly is set on an element with role grid, user agents must propagate the value to all gridcell elements owned by the grid and expose the value in the accessibility API. An author may override the propagated value of aria-readonly for an individual gridcell element.

In a grid that provides cell content editing functions, if the content of a focusable gridcell element is not editable, authors may set aria-readonly to true on the gridcell element. However, the value of aria-readonly, whether specified for a grid or individual cells, only indicates whether the content contained in cells is editable. It does not represent availability of functions for navigating or manipulating the grid itself.

An undefined value for aria-readonly, i.e., aria-readonly is not specified, does not imply that a grid or a gridcell contains editable content. For example, if a grid presents a collection of elements that are not editable, such as a collection of link elements representing dates in a datepicker, it is not necessary for the author to specify a value for aria-readonly.

Also:
While the columnheader role can be used in both interactive grids and non-interactive tables, the use of aria-readonly and aria-required is only applicable to interactive elements. Therefore, authors should not use aria-required or aria-readonly in a columnheader that descends from a table, and user agents should not expose either property to assistive technologies unless the columnheader descends from a grid.